It is known that a block copolymer can be obtained by an anionic copolymerization of a conjugated diene compound and an alkenyl arene compound by using an organic alkali metal initiator. Block copolymers have been produced which comprise primarily those having a general structure EQU A--B--A
wherein the polymer blocks A comprise thermoplastic polymer blocks of alkenyl arenes such as polystyrene, while block B is a polymer block of a conjugated diene such as butadiene and isoprene. The proportion of the thermoplastic blocks to the elastomeric polymer block and the relative molecular weights of each of these blocks is balanced to obtain a rubber having unique performance characteristics. In such a rubber, the blocks A are thermodynamically incompatible with the blocks B resulting in a rubber consisting of two phases; a continuous elastomeric phase (blocks B) and a basically discontinuous hard, glass-like plastic phase (blocks A) called domains. These domains act as physical crosslinks anchoring the ends of many block copolymer chains. Since the A--B--A block copolymers have two A blocks separated by a B block, domain formation results ineffectively locking the B blocks and their inherent entanglements in place by the A blocks and forming a network structure. Such a phenomena allows the A--B--A rubber to behave like a conventionally vulcanized rubber that contains dispersed reactive filler particles. These thermoplastic A--B--A rubbers are physically crosslinked by the domains in a network structure as opposed to being chemically crosslinked like a conventionally vulcanized rubber. As such, these polymers may be handled in thermoplastic forming equipment and are soluble in a variety of relatively low cost solvents. Additionally, polymers of this type are highly useful in that the vulcanization step is eliminated and, contrary to vulcanized scrap rubbers, the scrap from the processing of these thermoplastic elastomers can be recycled for further use.
These typed of block copolymers are diversified in characteristics, depending on the content of the alkenyl arene compound. When the content of the alkenyl arene compound is small, the produced block copolymer is a so-called thermoplastic rubber. It is a very useful polymer which shows rubber elasticity in the unvulcanized state and is applicable for various uses. For example, these polymers are applicable for uses such as moldings of shoe sole, etc.; impact modifier for polystyrene resins and engineering thermoplastics; in adhesive and binder formulations; modifiction of asphalt; etc.
Such block copolymers with a high alkenyl arene compound content, such as more than 70% by weight, provide a resin possessing both excellent impact resistance and transparency, and such a resin is widely used in the field of packaging. Many proposals have been made on processes for the preparation of these types of block copolymers (U.S. Pat. No. 3,639,517).
While in general these block copolymers have a number of outstanding technical advantages, one of their principal limitations lies in their sensitivity to oxidation. This behavior is due to the unsaturation present in the elastomeric section comprising the polymeric diene block. Oxidation may be minimized by selectively hydrogenating the copolymer in the diene block, for example, as disclosed in U.S. Pat. Re. 27,145. For example, prior to hydrogenation, the block copolymers have an A--B--A molecular structure wherein each of the A's is an alkenyl-arene polymer block and B is a conjugated diene polymer block, such as an isoprene polymer block or a butadiene polymer block which preferably contains 35-55 mole percent of the condensed butadiene units in a 1,2 configuration.
While these selectively hydrogenated block copolymers have vastly improved stability over their unsaturated precursors, they have certain shortcomings, which it would be desirable to eliminate or minimize. One such shortcoming is that these selectively hydrogenated block copolymers are deficient in many applications in which the retention of properties at elevated temperatures and deformation resistance are required. At relatively low temperatures, say room temperature, such block copolymers are known to have particularly high tensile strengths due to the formation of glassy phase arene block domains which act as physical crosslinks locking in the inherent entanglements within the rubbery B block matrix. The mechanical integrity of these domains and resulting network structure formed appear to control the tensile strengths of these copolymers. Moreover, at elevated temperatures, the mechanical integrity of block copolymers is limited to the integrity of the hard phase arene block domains. For example, network forming copolymers having arene blocks of polystyrene have poor mechanical properties at high temperature which may be attributed to the weakening of the polystyrene domains above its glass transition temperature (Tg) of 100.degree. C. Improvements in the high temperature characteristics of the network forming block copolymers may be achieved by enhancing the integrity of the alkenyl arene domains to higher temperatures.
These selectively hydrogenated block copolymers are further deficient as a result of their poor processability. It is possible, of course, to improve processability by diluting the polymer with extending oils and the like. This normally results in a drastic reduction in other physical properties, particularly, heat resistance, tensile strength and properties associated therewith. Blends of these block copolymers with a second resin for processability improvement are known, but in most instances the second resin is a relatively nonpolar polymer.
The foregoing accents a further deficiency of these selectively hydrogentated block copolymers. In particular, these selectively hydrogenated block copolymers are deficient in many applications in which interactions are required between it and other materials. Applications in which improvements in adhesion characteristics may promote improved performance include (1) the toughening of, and dispersion in, polar polymers such as the engineering thermoplastics; (2) the adhesion to high energy substrates in a hydrogenated block copolymer elastomer based high temperature adhesive, sealant or coating materials; and (3) the use of hydrogenated elastomers in reinforced polymer systems. The placement of functional groups onto the block copolymer may provide interactions not possible with hydrocarbon polymers and, hence, may extend the range of applicability of this material.
Though highly polar polymers typically are not compatible with these block copolymers, many attempts have been made to blend them just the same. Blends of polystyrene/polybutadiene/polystyrene block copolymer (SBS) with nylon polymers have been disclosed in U.S. Pat. No. 3,546,319 (15% SBS rubber in polyamide), Belgium Pat. No. 70,498 (35% nylon in SBS), and Japan Pat. No. 7,138,611 (5 to 50% SBS in polyamide). Additionally, blends of a selectively hydrogenated block copolymer having at least two monoalkenyl arene blocks and at least one selectively hydrogenated conjugated diene block there between with, for example (1) nylon (polyamide) polymers have been disclosed in U.S. Pat. No. 4,041,103 (100 parts by weight of a block copolymer with 5 to 200 parts by weight of a polyamide) and 4,242,470 (100 parts by weight of a polyamide with less than 50 parts by weight of a block copolymer); (2) polyesters have been disclosed in U.S. Pat. No. 4,101,605; (3) polyurethanes have been disclosed in U.S. Pat. Nos. 4,088,627 and 4,107,131; and (4) halogenated thermoplastic polymers have been disclosed in U.S. Pat. No. 4,096,204. However, the poor compatibility of the block copolymer component or its selectively hydrogenated counterpart with the polar polymer is not satisfactorily overcome in such blends and results in heterogenous or non-adhering polymer mixtures with their associated defects.
As earlier noted, the placement of functional groups onto the block copolymer may provide sites for interactions with such polar resins and, hence may extend the range of applicability of this elastomer. Such interactions, which include chemical reaction, hydrogen bonding and dipole interactions, are a route to achieving improved interfacial adhesion, hence improved compatibility with polar thermoplastics.
Many attempts have been made to improve compatibility with polar thermoplastic polymers by adding low modulus modifiers which contain polar moieties as a result of polymerization or which have been modified to contain polar moieties by various grafting techniques. To this end, various compositions have been proposed utilizing such modifiers having carboxylic acid moieties and derivatives thereof, for example, Epstein in U.S. Pat. Nos. 4,174,358 (polyamides) and 4,172,859 (polyesters); Saito et al. in U.S. Pat. No. 4,429,076 and in German Offenlegunsschrift 3,022,258 (published Jan. 8, 1981); Hergenrother et al. in U.S. Pat. No. 4,427,828 (polyamides); Harlan in U.S. Pat. No. 4,007,311 (polyurethane); and Shiraki et al. in U.S. Pat. Nos. 4,628,072; 4,657,970; and 4,657,971.
Epstein dicloses a broad range of low modulus polyamide and polyester modifiers which have been prepared by free radical copolymerization of specific monomers with acid containing monomers (60 to 99% polyamide or polyester and correspondingly 40-1% w modifier). Alternatively, Epstein discloses the modification of polymers by grafting thereto specific carboxylic acid containing monomers. The grafting techniques allowed for therein are limited to thermal addition (ene reaction) and to nitrene insertion into C-H bonds or addition to C.dbd.C bonds (ethylenic unsaturation). Though Epstein does disclose a broad range of polyamide and polyester modifiers, Epstein does not disclose or suggest the utilization of hydrogenated copolymers of alkenyl arenes and conjugated dienes nor, more particularly, modified selectively hydrogenated copolymers of alkenyl arenes and conjugated dienes as polyamide modifiers.
Saito et al. ('076 and '258) disclose polar thermoplastic compositions which contain a modified unsaturated aromatic vinyl compound/conjugated diene block copolymer (1 to 99 parts by weight polyamide and correspondingly 99 to 1 parts by weight modified block copolymer). The unsaturated block copolymer has been modified by grafting a dicarboxylic acid group or derivative thereof (e.g. anhydride moieties) at a point of ethylenic unsaturation via thermal addition (ene reaction). However, such modifiers and compositions containing same are deficient in that the weatherability and resistance to thermal deterioration are poor; and, therefore, the polymers and compositions have been used only in the fields where such properties are not required. Furthermore,it is also noted that the ene reaction is a reversible reaction.
Harlan discloses polyurethane compositions which contain a modified monoalkenyl arene/conjugated diene block copolymer whch may be unsaturated or selectively hydrogentated (5 to 50% by weight block copolymer and correspondingly 95 to 50% by weight polyurethane cement adhesive.) The block copolymer therein has been modified by grafting a polymerized alkyl ester of an acid of the acrylic acid series (e.g., esters of methacrylic acid) in the presence of a free radical catalyst such as an organic peroxide.
Hergenrother et al. (polyamide) and Shiraki et al. (polar thermoplastic polymer) also describe a composition containing a block copolymer similar to that of Saito et al. However, in order to improve the weatherability and resistance to heat aging, both partially hydrogenate the block copolymer in their respective blends to an ethylenic unsaturation degree not exceeding 20 percent of the ethylenic unsaturation contained in the block copolymer prior to hydrogenation. Once the block copolymer is partially hydrogenated, the block copolymer is modified by grafting a molecular unit containing a carboxylic acid group and/or a group derived therefrom (e.g. anhydride moieties). Hergenrother et al. disclose grafting via thermal addition (ene reaction) utilizing the available residual unsaturation in the block copolymer. As such, Hergenrother et al. retained the deficiencies associated with the reversibility of the ene reaction. On the other hand, Shiraki et al. utilized free radical initiators to perform the grafting therein. Additionally, Hergenrother, et al. limit their disclosure to blends containing the polyamide as the major component, i.e., 50 to 99%, polyamide and correspondingly 50 to 1% modifier block copolymer.
As is readily apparent in each of the foregoing prior art polar thermoplstic polymer compositions utilizing a modified alkenyl arene/conjugated diene block copolymers, improved compatability with the particular polar thermoplastic polymer is achieved via specific interactions, between the modified diene block and the polar thermoplastic polymer.
On the other hand, Gergen et al., in the copending U.S. patent application Ser. No. 766,215 which issued on Nov. 11, 1988 as U.S. Pat. No. 4,783,503 and 766,216 which issued on Jan. 10, 1989 as U.S. Pat. No. 4,797,447 describe a polyamide and a polyester composition, respectively, containing a block copolymer which is a thermally stable, modified, selectively hydrogenated, high 1,2 content alkenyl arene/conjugated diene block copolymer grafted with at least one functional group utilizing the metalation process. Therein, the functional groups are grafted primarly in the alkenyl arene block, thereby avoiding any adverse effects associated with modifying the diene block. In this composition, interactions between the polar thermoplastic polymer and rubber are achieved via the alkenyl arene block. The compositions therein and resinous in nature, i.e., 50-97% w polymide or polyester and correspondingly 50-3% w modifier block copolymer.
Further research and experimentation on polar thermosplastic polymer compositions containing the modified block coolymers utilized in Gergen et al. in copending U.S. patent applications Ser. Nos. 766,215 which issued on Nov. 11, 1988 as U. S. Pat. No. 4,783,503 (K-4669) and 766,216 which issued on Jan. 10, 1989 as U.S. Pat. No. 4,797,447 K-4801) have yielded unexpected and significant improvements in tensile strength (particularly at high temperatures) and oil resistance. These new polar thermoplastic polymer blend compositions contain block copolymers having the carboxyl functional groups present in their acid, ester and/or neutralized metal carboxylate salt forms. In particular, the improvement is believed to increase as the proportion of carboxyl functional groups in their acid form increases. Whether either or both of these forms in combination produce optimum performance may be dependent on the particular polar thermoplastic polymer selected. Furthermore, the tensile strength and high temperature properties ar also improved by increasing the degree of carboxyl functionality.
To those skilled in the art, the degree to which the grafting reaction and/or strong physical mutual interaction phase size reduction occur, thereby promoting interfacial adhesion, together with the distribution of the polar thermoplastic polymer within the blend typically contribute to the tensile strength of the blend. The results herein demonstrate that functionalizing the alkenyl arene segment promotes covalent bonding between the modified block copolymer and the polyamide. Furthermore, the polar thermoplastic polymer also becomes well distributed in the block copolymer phase.